Solvatochromic solvent properties scale parameters

Tfaeie have been a number of attempts to develop solvent parameter scales that could be used to correlate ttiermodynamic and kinetic results in terms of these patametois. Gutmann s Donor Numbers, discussed previously, are sometimes used as a solvent property scale. Kamlet and Taft and co-workers developed the solvatochromic parameters, Uj, B, and n that are related to the hydrogen bonding acidity, basicity and polarity, respectively, of the solvent. Correlations with these parameters also use the square of tte Hildebrand solubility parameter, (5, that gives the solvent cohesive energy density. Parameters for some common solvents are collected in Table 3.6. 

Over the last few years, the development of solvents of desired properties with a particular use in mind has been challenging. To evaluate the behaviour of a liquid as solvent, it is necessary to understand the solvation interactions at molecular level. In this vein, it is of interest to quantify its most relevant molecular-microscopic solvent properties, which determine how it will interact with potential solutes. An appropriate method to study solute-solvent interactions is the use of solvatochromic indicators that reflect the specific and non-specific solute-solvent interactions on the UV-Vis spectral band shifts. In this sense, a number of empirical solvatochromic parameters have been proposed to quantify molecular-microscopic solvent properties. In most cases, only one indicator is used to build the respective scale. Among these, the E (30) parameter proposed by Dimroth and Reichardt [23] to measure solvent dipolarity/polarisability which is also sensitive to the solvent s hydrogen-bond donor capability. On the other hand, the n, a and P (Kamlet, Abboud and Taft)... 

The basic premise of Kamlet and Taft is that attractive solute—solvent interactions can be represented as a linear combination of a nonspecific dipolarity/polarizability effect and a specific H-bond formation effect, this latter being divisible into solute H-bond donor (HBD)-solvent H-bond acceptor (HB A) interactions and the converse possibility. To establish the dipolarity/polarizability scale, a solvent set was chosen with neither HBD nor HBA properties, and the spectral shifts of numerous solvatochromic dyes in these solvents were measured. These shifts, Av, were related to a dipolarity/polarizability parameter ir by Av = stt. The quantity ir was... 

The dielectric constant and refractive index parameters and different functions of them that describe the reactive field of solvent [45] are insufficient to characterize the solute-solvent interactions. For this reason, some empirical scales of solvent polarity based on either kinetic or spectroscopic measurements have been introduced [46,47]. The solvatochromic classification of solvents is based on spectroscopic measurements. The solvatochromic parameters refer to the properties of a molecule when its nearest neighbors are identical with itself, and they are average values for a number of select solutes and somewhat independent of solute identity. 

The p scale was proposed to measure solvent hydrogen bond basicity, i.e. the ability of a bulk solvent to act as hydrogen bond acceptor. This scale was derived by systematic application of the solvatochromic comparison method the final p values were calculated by averaging 13 p parameters for each solvent obtained with different solutes and different physicochemical properties [Kamlet et al, 1981a Kamlet et al, 1983]. 

Ionic liquids are however more just than a bulk medium and the dielectric constant may be not the best parameter to define ILs polarity. They are constituted by positive and negative ions which can exert various effects. Recently, the microscopic properties of ILs, i.e. the ability of these media to interact with specific dissolved species (reagents, transition states, intermediates and products), have been measured and several polarity scales, previously developed for common molecular solvents, have been extended to ILs. At variance with molecular solvents, ILs are characterized by complex interaction forces between anion and cation and these interactions are competitive with the ability of both anion and cation to interact with dissolved species thus, multiparameters solvatochromic correlations, better than single point measurements, resulted useful to understand the solvent polarity. ... 

One such methodology is the Kamlet-Taft Solvatochromic parameter approach. In this methodology, a solvent can be characterized by three parameters, tt, a measure of the polarity and polarizability of the fluid, a, the acidity or hydrogen bond donor capability and P, the hydrogen bond acceptor capability or basicity. Each of these parameters is determined from the shift in UV-visible absorbance of a series of select indicator species dissolved in the solvent. Rather than depending on the bulk properties of the fluid, as is the case with the cohesive energy approaches, the solvatochromic parameters are derived from the interactions between the indicator solute and the immediate solvent shell, in effect they are a measure of how a solute sees the solvent. In each case, the scale of values has been normalized to between 0.0 for cyclohexane... 

The following treatment is based on the use of three different scales [i.e., (S), g2(S), and g3(S)] which have been determined empirically the polarity scale 7T, the a scale of solvent hydrogen bond donor (HBD) acidities (71), and the /3 scale of solvent hydrogen bond acceptor (HBA) basicities (72). To avoid possible pitfalls resulting from experimental errors or from specific solvent effects, the solvatochromic parameters have been arrived at by averaging multiple, s determined for each solvent with a variety of different indicators. Quite generally, the purpose of this study is the systematic correlation of solvent effects on diverse properties and reactivity parameters, XYZ, by means of expressions of the type. 

To adapt the solvatochromic equations and the v scale to properties and phenomena involving different relative contributions of polarity and polarizability, we have modified Equations 42,42 and 89 by the addition of a d5 term. The d coefficient is intended to serve as a measure of the difference between the polarity- polarizability blend for XYZ and the rr blend. The 5 parameter is taken to be 0.00 for all select solvents, 0.50 for polychlorinated aliphatics, and 1.00 for all aromatic solvents. 

Because of the often-observed inadequacies of the dielectric approach, that is, using die dielectric constant to order reactivity changes, the problem of correlating solvent effects was next tackled by the use of empii ical solvent parameters measuring some solvent-sensitive physical property of a solute chosen as the model compound. Of these, spectral properties such as solvatochromic and NMR shifts have made a spectacular contribution. Other important scales are based on enthalpy data, with the best-known example being the donor number (DN) measuring solvent s Lewis basicity. 

This donor number scale is widely referenced in relation to thermodynamic properties, as well as electron-transfer kinetics and photochemical properties. It has been criticized because of the neglect of solvent effects and side reactions that contribute to and because a one-parameter scale can never be entirely adequate. Ambiguities can arise for solvents which have more than one donor site, such as the formamide and sulfoxide derivatives. Recent measurements with BFj as the acid have provided some points of comparison and criticism for the original donor numbers. Recently, Linert et al. have used the solvatochromic shifts of a Cu(II) complex to define donor numbers for anions in dichloromethane. They also have suggested how these values can be converted for use in other solvents through a correlation with the acceptor number of the solvent. Linert et al. have reviewed the area and provided an extensive compilation of donor numbers from calorimetric and solvatochromic shift measurements. Some anion donor numbers in dichloromethane are included in Table 3.4, and the values for anions in water are 21 kcal moH smaller than those given. 

Among the approaches proposed so far, we recall here single-parameter models [102-111, 115, 118-120, 122, 123, 125, 126, 129], and multi-parametric correlation equations (either based on the combination of two or more existing scales or on the use of specific parameters to account for distinct types of effects) [112, 113, 116, 117, 121, 124]. Additional popular models are the Abraham s scales of solute hydrogen-bond acidity and solute hydrogen-bond basicity [127, 128], and the Catalan et al. solvatochromic scales [130,132, 133]. Methods based on quantitative stmcture-property relationships (QSPR) with solvent descriptors derived from the molecular structure [131, 134], and on principal component analysis (PCA) [135, 136] have been also proposed. An exhaustive review concerning the quantification of the solvent polarity has been recently published [138-140], including a detailed list of solvent scales, interrelations between parameters and statistical approaches. 

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